Motor vehicles which are equipped with an internal combustion engine comprise fuel tanks which store fuel required for the operation of the internal combustion engine. For internal combustion engines which operate according to the four-stroke principle, petrol is accordingly stored in the fuel tank. Conventional liquid petrol consists of a mixture of various hydrocarbons, of which the evaporation temperatures are in the range of normal ambient temperatures or lower temperatures. Therefore, a proportion of the fuel is evaporated when liquid petrol is stored in the fuel tank. In addition to the liquid petrol, there is therefore also an evaporated, gaseous fuel in the fuel tank. This effect occurs with all fuels which display a tendency to evaporate under the normal operating conditions of the engine. The on-going evaporation of the liquid fuel at corresponding temperatures leads to pressure increases in the fuel tank, which is counteracted by various means. According to current emission regulations, compliance with which is determined by the mandatory SHED test, these gaseous hydrocarbons must, however, be prevented from entering the environment and thereby causing damage to health and the environment. In order to prevent high component costs, the fuel tank is not configured as a pressure vessel, but rather is only dimensioned up to a relative pressure difference of approximately +/−50 mbar. In order to prevent pressure loads, pressure-equalising systems are used. For this, in conventional motor vehicles, in addition to tank-aeration systems, tank-deaeration systems are also provided, in which the gaseous fuel is conducted out of the fuel tank, and an increase in pressure is thus prevented. The fuel which is conducted out is then stored temporarily in, for example, an activated carbon canister and, during the operation of the internal combustion engines, is fed into the combustion via corresponding connections, devices and control systems. According to the storage capacity of these activated carbon canisters, certain flushing cycles are required in which the fuel stored in the activated carbon canister is flushed out. The flushing can either take place passively, at certain operating points of the internal combustion engine, or actively, by using additional pumps. If the activated carbon canister cannot be flushed because the motor vehicle for example is stationary for a relatively long time, the maximum intake capacity is reached. The filter effect of the activated carbon filter is lost, and evaporated fuel is released into the environment. This is generally known as breaking through the activated carbon filter. Alternatively, systems are also known which liquefy the gaseous fuel which is conducted out using cooling systems and feed the condensed fuel back into the fuel tank. The effect of the tank-deaeration system is shown especially during the heating of the petrol due to high ambient temperatures, due to waste heat during the operation of the internal combustion engine, and due to mechanical losses in the fuel supply, such as in the fuel pump for example. Emissions which occur due to the motor vehicle being stationary for long periods of time or when refuelling can also be prevented.
If, however, further drive units such as electromotive drives are provided in addition to the internal combustion engine, the flushing cycles change. If the electromotive drive is used more frequently or is given priority during the operation of the motor vehicle, fewer cycles for flushing the activated carbon canister are available. If the internal combustion engine is only used as an ancillary unit, in order for example to increase the operating range of the motor vehicle, it must be constantly ensured that said engine is ready for use, and a sufficient quantity of fuel must be carried. In that case, the problem is exacerbated further, since operation of the internal combustion engine is only required in exceptional cases. The internal combustion engine is then only operated at optimal-efficiency operating points at which the conventional tank deaeration cannot work. However, emissions which occur due to evaporated fuel must also be prevented in this case. The fuel tank can be configured as a pressure vessel, but this has proven to be disadvantageous in terms of component costs, component weight and also, as a result of the high vessel pressure, in terms of refuelling. Conventional systems with temporary storage and subsequent feeding to the combustion in internal combustion engines are not suitable therefor. In addition, the laid-open application DE 10 2008 047 721 A1, for example, discloses systems which temporarily store the gaseous hydrocarbons in a reserve tank which is designed as a pressure vessel. The laid-open application DE 10 2007 051 469 A1 discloses a fuel tank comprising a deaeration line having a condensation means, wherein the condenser cools and thus liquefies the gaseous fuel. As a result, it is possible to conduct the liquefied fuel back into the fuel tank. The condenser is operated using a coolant from an absorption or compression refrigeration system.
The laid-open application DE 27 56 348 A1 discloses an absorption refrigeration system by means of which the temperature in a passenger compartment and payload space of a motor vehicle is controlled. In an advantageous configuration, the fuel is additionally cooled by means of a fuel cooler. In this case, the absorption refrigeration system is powered by waste-heat-generating drive units of the motor vehicle. The waste heat from the internal combustion engine itself and the waste heat from the oil from gears or retarders are disclosed as heat sources.
DE 32 14 874 C2 discloses a device for cooling fuel in the fuel tank. In this case, a heat exchanger serving as an evaporator or a heat exchanger which is connected downstream of an evaporator is installed in the fuel tank.
Cooling, liquefying and storage systems which are integrated in the fuel tank or in fuel return lines and operate using electrical energy or draw energy from the operation of an internal combustion engine are also unsuitable in view of the optimisation of the operating range of electric and hybrid vehicles.